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Feeding solar power back into municipal grid: Issues and finger-pointing - Page 33

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Posted by Home Guy on April 16, 2011, 1:42 pm
 
harry wrote:


That's exactly what I've been saying - that you "turn it up" (the
inverter's voltage output) to maximize the PV's current (I) supply into
the grid.  

But everyone else (or most everyone else) is saying no - that simply
matching the grid voltage (as measured at your service connection) is
all that happens (and is all that needs to happen) for the entire PV
current (I) capacity of the PV system to be "injected" into the grid.

So now that we agree that PV systems need to raise the grid voltage if
they're going to "force" their maximal available supply capacity into
the grid, it's a moot or academic question as to what exactly their
supply situtation would be (how much current they'd supply into the
grid) if the invertors simply matched the grid voltage.

Posted by Jim Wilkins on April 16, 2011, 2:36 pm
 

You can only guess at what the inverter is forcing and sensing without
looking at the schematic and uP code or a technical explanation of it.
The Wiki type explanations are oversimplified for general readers,
engineers have better sources.

I was hired to decipher and troubleshoot several lead-acid and lithium
battery charging circuits after the designers quit, and found a couple
of different approaches in use. Generally they compared voltage and
current measurements to a model and used the result to pulse-width-
modulate the output control.

Power factor control is similar to the design issues of a grid-tie
inverter, with a large enough market to support custom ICs:
http://focus.ti.com/lit/an/slua144/slua144.pdf

 jsw

Posted by krw@att.bizzzzzzzzzzzz on April 16, 2011, 6:39 pm
 

ICs:http://focus.ti.com/lit/an/slua144/slua144.pdf

Motors aren't the only things that need PF correction.  Capacitors aren't the
only, or often the, way of doing it.

Posted by Jim Wilkins on April 16, 2011, 10:13 pm
 
An incorrect assumption. Those are for more complex loads like
switching power supplies with rectifier inputs.

jsw

Posted by g on April 12, 2011, 5:39 pm
 On 12/04/2011 07:33, Home Guy wrote:

1) The actual voltage increase will relate to the ratio of grid
impedance vs local impedance, i.e. your local power consumers (fridges,
heaters etc) has a much higher impedance relatively, thus the grid will
"take" the majority of the generated power. The _only_ increase in
voltage you will see results from the voltage drop in the grid components.


2) Pretty complex calculation, but yes, _somewhere_ one or more
generating pieces of machinery will reduce its output. Makes sense
intuitively, does it not?


3) You just set your PV system to operate at max power, the grid system
will balance out automatically. See 1) above


4) The grid voltage does actually fluctuate a bit, depending on load.
Power companies have means of adjusting line voltages depending on load
fluctuations. The average subscriber never knows this.

5) That will be a very inefficient way to utilize your PV system.

A simplified way is to look at the grid as a battery. When your PV
system generates more power than your local consumers, the surplus will
flow into the grid. At all other times the grid and the PV will both
supply the needed power to the local consumers.


6) Fairly close to impossible. How do you match local power consumers to
hit the 100% PV capacity?


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